Gas chromatography (GC) is used to separate and analyze compounds that can be vaporized without undergoing degradation. Typically, a sample containing a number of components to be analyzed is moved through a column via a carrier gas. The packing material, or walls of the column contain a chemical(s) that interacts with the components in the sample, such that different components move through the column at different rates of speed. Hence, the different components of the sample are eluted from the sample at different times even though all of the components entered the column at the same time. The performance of the GC depends on a mechanism for introducing the sample into the gas flow over a short period of time without significantly disturbing the gas flow.
For samples that are not gases at room temperature, a mechanism is needed to heat the sample to the vapor phase and then introduce that gas into the GC column over a short period of time. Injection ports on GCs are maintained at an elevated temperature so that when a liquid sample is injected, the solvent and sample(s) dissolved in the solvent, are vaporized. There is a balance between having the injection temperature high enough to vaporize the sample rapidly, but not too high as to thermally degrade any of the components of the sample. Liquid samples are typically injected into the injection port with a syringe and needle. The needle of the syringe pierces the septum on the injection port and the plunger of the syringe is depressed, forcing the liquid sample into the injection port. Automated injectors generally provide more consistent results than injection schemes based on manual injection of the sample. However, neither of these schemes lends itself to the processing of solid samples. In addition, injection ports will typically discriminate during the injection process. That is, lower boiling point solvents and sample(s) will vaporize earlier than higher boiling point compounds. Also, depending on the injection technique and injection port design, the solvent and sample(s) will vaporize at different times, resulting in a broader than desired sample solvent plume entering the column. As a result, different injectors are required for different types of samples and different types of analysis.
Injection techniques based on samples that have been previously packaged into capsules, in principle, can overcome many of these problems. In such systems, the sample is placed in a capsule and sealed. The sealed capsule is introduced into a chamber in the GC where it is heated to the desired temperature to volatilize the sample and then opened quickly in the gas stream. This arrangement allows a single type of injector to handle different types of samples. In addition, only the capsule loading apparatus need be located at the sampling site, and hence, the problems associated with remote monitoring are greatly reduced. Unfortunately, prior art capsules for use in such systems have had significant problems that limit this type of GC system.
The ideal capsule must satisfy a number of criteria. First, the capsules must be capable of being loaded and sealed without vaporizing the sample being loaded into the capsule. Second, the sealing mechanism must be easily implemented in a cost-effective manner. Third, the capsules must be chemically inert and non-volatile at the temperatures used to vaporize the samples. Fourth, the capsules must be easily punctured within the GC to release the volatile contents of the capsule. Finally, the spent capsules must be easily removed from the GC.
Unfortunately, prior art capsules differ significantly from this ideal, which has limited their use. Prior art capsules for use in GCs are typically constructed from metal or a glass. Metal capsules constructed from gold, indium, or aluminum, are known to the art. Metal capsules present problems in terms of sealing the sample in the capsule. In general, the capsule end is either closed by a crimping operation or welded shut after introducing the sample into the capsule. Crimp seals can appear closed even when the seal has a small leak. For a solid or liquid sample, the leak may not become evident until the capsule is heated in the GC. Such a capsule will release the contents over an extended period of time before the capsule is actually punctured. Welding also presents problems. Since the metals are good heat conductors, the heat generated by the welding operation at one end of the capsule can be transferred to the sample during the welding operation and cause part of the sample to vaporize and escape before the weld is complete.
Most metals are not chemically inert at the temperatures at which the samples are vaporized. Those that are relatively inert, such as gold, are expensive. In addition, metals that melt at the injection temperature leave material in the inlet that is difficult to remove.
Glass capsules provide improvements in terms of chemical inertness and cost. However, these capsules are difficult to seal and leave small pieces of glass behind after opening that are difficult to remove from the inlet to the GC. Glass capsules are typically glass capillaries with inner diameters of around 1 mm and closed on one end. Samples are placed in the glass capillary and then sealed with a flame. The sealed glass capillary is placed in the heated zone of the inlet to the GC and allowed to come to temperature. Once at temperature, the capillary is broken and the volatile contents released and the carrier gas would sweep the volatiles into the inlet to the GC.
Sealing of the glass capillary using a flame requires expertise on the part of the user and is not easily automated. Furthermore, during the sealing process, the capillary is heated, and heat can be transferred to the sample causing volatiles to escape. Another problem with sealing the glass capsule using heat or flame is that even when the end of the capsule is sealed, the glass remains at an elevated temperature and in a softened state for some time. The residual heat can be transferred to the solvent in the capsule causing the solvent to vaporize and pressurize the capsule leading to the softened glass seal failing. Finally, as noted above, when the glass capillary is broken in the inlet, small pieces of glass that are difficult to remove are generated.